Tap-changing series-multiple transformer system

ABSTRACT

A transformer system providing proper tap changing in both the series and the multiple connection arrangements. In series operation, the regulating voltage is supplied by a regulating winding through which all of the load current flows. In multiple operation, the regulating voltage is supplied across a combination of the regulating winding and an autotransformer. The autotransformer is tapped at a position which provides the desired regulating voltage corresponding to the normal series and parallel voltage ratio. A portion of the load current flows through the portion of the autotransformer winding which is on one side of the tap. The remaining portion of the load current flows through the portion of the autotransformer winding which is on the other side of the tap and through the regulating winding of the main transformer.

United States Patent Golaski et al.

[ June 18, 1974 TAP-CHANGING SERIES-MULTIPLE TRANSFORMER SYSTEM Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

Filed: May 30, 1973 Appl. No.: 365,367

[56] References Cited UNITED STATES PATENTS 5/1955 Willis et a1. 323/435 R 11/1960 Sealey 323/49 X 8/1963 Nielsen 323/435 R 6/1968 Schoendube et a1 .1 323/435 R Primary ExaminerThomas J. Kozma Attorney, Agent, or Firm-J. R. Hanway 7 1 ABSTRACT A transformer system providing proper tap changing in both the series and the multiple connection arrangements. In series operation, the regulating voltage is supplied by a regulating winding through which all of the load current flows. ln multiple operation, the regulating voltage is supplied across a combination of the regulating winding and an autotransformer. The autotransformer is tapped at a position which provides the desired regulating voltage corresponding to the normal series and parallel voltage ratio. A portion of the load current flows through the portion of the autotransformer winding which is on one side of the tap. The remaining portion of the load current flows through the portion of the autotransformer winding which is on the other side of the tap and through the regulating winding of the main transformer.

5 Claims, 6 Drawing Figures PATENTED JUN 1 81874 PRIOR ART PRIOR ART MAIN TRANSFORMER TERMINAL BOARD LOAD TAP CHANGER AUTO TRANSFORMER FIG.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to electrical inductive apparatus and, more specifically, to tap-changing, series-multiple transformer systems.

2. Description of the Prior Art Series-multiple transformer systems are used when it is desirable to be able to change the normal voltage rating of the system conveniently. When the transformer system is regulated by the use of a load tap changer, a separate transformer is usually placed in series with the main transformer to supply the regulated component of the output voltage.

In a series-multiple transformer system using a series transformer to supply the regulating voltage, the entire secondary winding of the series transformer carries the full load current when the system is connected for series operation. Thus, a series transformer which can develop 10 percent of the normal output voltage must have a KVA rating equal to approximately 10 percent of the main power transformer KVA rating. In such transformer systems suitable for use in power distribution systems, the size and cost of a series transformer having three or more windings adds considerably to the size and cost of the equipment and operation.

A series transformer for series-multiple operation must contain at least three windings. One primary and at least two secondary windings are required to permit the connections which provide the proper primary and secondary turns ratios. The need for a multiple-winding series transformer also adds considerably to the cost of the overall system.

Therefore, it is desirable, and it is an object of this invention, to provide a tap-changing, series-multiple transformer system which does not require a multiplewinding series transformer having a KVA rating which is equal to the product of the regulating voltage and the full load current in series operation.

SUMMARY OF THE INVENTION There is disclosed herein a new and useful tapchanging, series-multiple transformer system which utilizes a main power transformer and a single-winding autotransformer having a KVA rating which is less than the KVA rating of conventional prior art series transformers for the same output conditions. In series operation, the load current travels through a regulating winding of the main power transformer. The autotransformer is not actively connected in the circuit during series operation. In multiple operation, the autotransformer is connected across the regulating winding and a portion of the load current travels through the regulating winding and the autotransformer. The winding of the autotransformer is tapped for the connection of the load. The position of the tap depends upon the ratio of the series and multiple voltage ratings. Since the winding of the autotransformer carries only a portion of the load current through the winding portions located on each side of the output tap, the KVA rating of the autotransformer is less than the required KVA rating for a series transformer connected according to the prior art.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawing, in which:

FIG. 1 is a schematic diagram illustrating a seriesmultiple transformer system connected for series operation according to the prior art;

FIG. 2 is a schematic diagram illustrating a seriesmultiple transformer system connected for multiple operation according to the prior art;

FIG. 3 is a schematic diagram illustrating a seriesmultiple transformer system connected for series operation according to this invention;

FIG. 4 is a schematic diagram illustrating a seriesmulitple transformer system connected for multiple operation according to this invention;

FIG. 5 is a block diagram of a series-multiple transfomier system constructed according to this invention; and

FIG. 6 is a partial schematic of a series-multiple transfonner system constructed according to this invention wherein the series voltage is equal to three times the multiple voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar elements or members in all the figures of the drawing.

Referring now to the drawing, and to FIG. 1 in particular, there is shown a schematic diagram of a seriesmultiple transformer system constructed according to the prior art. The system is connected for series operation and includes a main power transformer 10, a series transformer 12, and a load tap changer represented in abbreviated form by the movable contact 14. The secondary windings 16, 18, 20 and 22 are all connected in series. The primary winding 24 of the series transformer 12 is connected to the tapped regulating winding 26 of the main power transformer 10. The series transformer 12 either adds to or subtracts from the voltage supplied by the main power transformer 10, depending on the polarity of the tap changer connections.

For consistency in this description of the preferred embodiments, the output voltage between terminals 26 and 28 and the range over which it may be changed will be given the same values for the prior art system and for the system constructed according to this invention. Any other values may be used without departing from the scope of the invention. In addition, a series-multiple transformer may have more than two tapped secondary windings to permit the desired voltage ratios. The specific arrangement of two secondary windings which may be connected in series or parallel is generally known in the art as a series-parallel arrangement.

In the series connection arrangement of FIG. 1, the output voltage is normally 8,800 volts and may be varied by plus or minus 10 percent, or from approximately 7,920 volts to 9,680 volts. Assuming three similar phases, the full load rating will be established at 12,500 KVA for these examples. In FIG. I, the three-winding series transfonner 12 must supply up to 10 percent, or 1,250 KVA, of the full load KVA rating.

FIG. 2 illustrates schematically a series-multiple transfomier system constructed according to the prior art and connected for multiple, or parallel, operation. Using the same components used in FIG. 1, the output voltage is normally 4,400 volts and may be varied by plus or minus percent, or from approximately 3,960 volts to 4,840 volts. The secondary windings l6, 18, and 22 are connected in parallel as shown to provide the desired output voltage. As in the series arrangement of FIG. 1, the series transformer 12 requires a component rating of 1,250 KVA to safely carry the full load output current at the parallel voltage level.

The invention disclosed herein provides the same output voltage range provided by the prior art with apparatus having a component KVA rating of 625 KVA. FIG. 3 shows schematically a series-multiple transformer system constructed according to this invention and connected for series operation. The main power transformer 10 includes the primary winding 30 and the magnetic core 32. Windings 16 and 18 are constructed and rated similar to the same windings discussed in connection with FIGS. 1 and 2. Each of the windings l6 and 18 has a voltage rating of 4,400 volts. Winding 34 has a voltage rating approximately equal to 10 percent of the total series voltage of 8,800 volts. With the polarity reversing and tap selecting capabilities of the tap changer, the normal output voltage between terminals 26 and 28 may be varied by plus or minus 10 percent as with the prior art arrangement. The KVA rating of the winding 34 is the same as the winding 26 shown in FIG. 1. The autotransformer 36 is a component of the series-multiple transformer system but is idle during the series connection arrangement.

FIG. 4 shows schematically the series-multiple transformer system of FIG. 3 connected for parallel operation. Secondary windings l6 and 18 are connected in parallel. The input terminals 38 and 40 and the output terminal 42 of the autotransformer 36 are connected as shown. The autotransformer 36 effectively reduces the voltage across the tapped regulating winding 34 to provide the suitable regulating voltage. With the arrangement illustrated, the autotransformer 36 must reduce the effect of the voltage across winding 34 on the output voltage by a factor of one-half.

The voltage across the regulating winding 34 is 20 percent of the normal output voltage in the parallel connection arrangement. However, the full load current in parallel operation cannot be carried safely by the regulating winding 34. Assuming the voltage tapped from the regulating winding 34 equals voltage V, voltage V/2 is developed across each half of the the centertapped autotransformer 36 and adds to or subtracts from the normal voltage depending on the polarity provided by the tap changer.

The voltage W2 is developed across each half of the winding 44 of the autotransformer 36. Each half of the winding 44 carries one-half of the full load current. The maximum value for the voltage V would be the entire voltage across the winding 34, or with the example values used, 880 volts. Therefore, the actual KVA rating of the autotransformer 36 must be proportional to the product of 10 percent of the normal series connection voltage and one-half the full load current in the parallel connection. Thus, with the series-multiple transformer system of this invention, the 1,250 KVA three-winding series transformer 12 as used in the prior art can be replaced, with the proper connections, by a two-part single-winding autotransformer 36 and still provide the same degree of voltage regulation with half of the total parts KVA rating. Since the series transformer 12 has both primary and secondary windings, the amount of copper conductors required is approximately two times that required in the autotransformer 36. A reduction in the magnetic core material required is also realized with the autotransformer 36.

Changing from series to parallel operation requires a change in various connections between the components of the transformer system. FIG. 5 is a block diagram illustrating a convenient arrangement for making the connections. The secondary windings of the main transformer 10 are connected to a terminal board 48 as are the load tap changer 50 and the autotransformer 36. The tapped regulating winding 34 of the transformer 10 is also connected to the tap changer 50. The terminal board 48 may contain the appropriate termi nals or lugs which must be interconnected to provide the desired operation.

The series-multiple transformer system disclosed herein is not limited to systems wherein the ratio between the different voltages is equal to a factor of two. FIG. 6 shows, in a partial schematic, an autotransformer arrangement for providing a regulated series voltage which is three times the parallel voltage. The output terminal 54 of the autotransformer 52 is tapped to the winding 56 in such a manner that one-third of the total electrical turns in the winding 56 are located between the input terminal 58 and the output terminal 54. Thus, in parallel operation, the output voltage is regulated by one-third of the voltage tapped from the regulating winding 34. One-third of the load current flows through the portion of the winding 56 above the output terminal and the other two-thirds of the load current flows through the portion of the winding 56 below the output terminal and through the regulating winding 34. Although winding 44 is illustrated as a single tapped winding, using two windings serially connected together with the appropriate terminal connections is within the contemplation of this invention.

Due to the lower total KVA requirements, a seriesmultiple transformer system constructed according to this invention permits a significant reduction in the cost of the system components over prior art arrangements. Since numerous changes may be made in the above described apparatus, and since different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawing, shall be interpreted as illustrative rather than limiting.

We claim as our invention:

1. A tap-changing, series-multiple transformer system comprising:

a main power transformer having at least first and second secondary windings and a tapped regulating winding;

an auxiliary autotransformer winding having first and second input terminals and an output terminal;

means for providing a series and a multiple connection arrangement of said secondary and regulating windings and said autotransformer;

said means connecting, in the series connection arrangement, said first and second secondary windings and at least a portion of said regulating windings all in series circuit relationship; and

said means connecting, in the multiple connection arrangement, said first and second secondary windings in parallel circuit relationship, said autotransformer input terminals across at least a portion of said regulating winding, and one of said autotransformer input terminals to the parallel combination of said secondary winding.

2. The tap-changing, series-multiple transformer system of claim 1 wherein the means for providing series and multiple connection arrangements comprises a load tap changer connected to tap terminals of the regulating winding, and a terminal board electrically connected to the main power transformer, the tap changer, and the autotransformer.

3. The tap-changing, series-multiple transformer system of claim I wherein the transformer system has a series secondary voltage rating equal to approximately two times the multiple voltage rating, and wherein the output terminal is connected in center-tapped relationship to the autotransformer.

4. The tap-changing, series-multiple transformer system of claim 1 wherein the autotransformer has a voltampere rating approximately equal to one-half the product of the voltage across the regulating winding and the normal full load current rating of the transformer system when connected in the multiple connection arrangement.

5. The tap-changing, series-multiple transformer system of claim 1 wherein the output terminal of the autotransformer is connected to a tap terminal on a winding of the autotransformer, with the ratio of the portion of the autotransformer winding tapped by the output terminal being approximately equal to the ratio of the multiple and series voltage ratings of the transformer system. 

1. A tap-changing, series-multiple transformer system comprising: a main power transformer having at least first and second secondary windings and a tapped regulating winding; an auxiliary autotransformer winding having first and second input terminals and an output terminal; means for providing a series and a multiple connection arrangement of said secondary and regulating windings and said autotransformer; said means connecting, in the series connection arrangement, said first and second secondary windings and at least a portion of said regulating windings all in series circuit relationship; and said means connecting, in the multiple connection arrangement, said first and second secondary windings in parallel circuit relationship, said autotransformer input terminals across at least a portion of said regulating winding, and one of said autotransformer input terminals to the parallel combination of said secondary winding.
 2. The tap-changing, series-multiple transformer system of claim 1 wherein the means for providing series and multiple connection arrangements comprises a load tap changer connected to tap terminals of the regulating winding, and a terminal board electrically connected to the main power transformer, the tap changer, and the autotransformer.
 3. The tap-changing, series-multiple transformer system of claim 1 wherein the transformer system has a series secondary voltage rating equal to approximately two times the multiple voltage rating, and wherein the output terminal is connected in center-tapped relationship to the autotransformer.
 4. The tap-changing, series-multiple transformer system of claim 1 wherein the autotransformer has a volt-ampere rating approximately equal to one-half the product of the voltage across the regulating winding and the normal full load current rating of the transformer system when connected In the multiple connection arrangement.
 5. The tap-changing, series-multiple transformer system of claim 1 wherein the output terminal of the autotransformer is connected to a tap terminal on a winding of the autotransformer, with the ratio of the portion of the autotransformer winding tapped by the output terminal being approximately equal to the ratio of the multiple and series voltage ratings of the transformer system. 